1. Field of the Invention
The present invention relates to cellulosic substrates, such as paper sheets, that are rendered water resistant and useful for photocopying, printing, and writing, by coating with a chemical composition, as well as to methods for making such products, and to products made by such methods.
2. Description of the Related Art
Various methods are known for treating paper to make it liquid-repellant or liquid-proof, or to enhance its wet-strength. Often, the liquid of interest is water or an aqueous solution. Paper has been made water-repellent, that is, resistant to wetting, most often to prevent spreading or “feathering” of ink applied to the paper. This is commonly referred to as “sizing.” Paper, such as that used for milk and juice containers, has been rendered waterproof by providing an impermeable barrier in the form of a film or coating that may cover the pores of the paper, as well as the fibers. The wet-strength of paper, such as that used for tissue, paper towels, and filters, has also been enhanced, generally by providing interfiber bonds that are more resistant to attack by water, rather than preventing water from permeating paper fibers.
Generally, it is advantageous to provide water resistance to paper used for printing or writing by coating or impregnating a surface with a suitable material. Such a method allows the use of a variety of stock papers already available from paper manufacturers.
A number of known methods for treating paper to render it more water-repellent, are concerned with paper sizing. Surface sizing is the application of a non-pigmented sizing agent as a coating to the surface of paper, while internal sizing entails the addition of sizing agents to paper pulp before it is formed into sheets. In addition to referring to a method for smoothing a paper surface and delaying or preventing the absorption and feathering of liquids, such as printing and writing inks, applied thereon, sizing may also, as one example, refer to a method for coating milk carton stock to prevent any fluid flow through its walls and edges.
Various compositions have been described as suitable for surface sizing. U.S. Pat. No. 6,290,814 B1 suggests surface sizing paper using a combination of gellan gum (a heteropolysaccharide) and derivatized starch, as an improvement over older methods using agents such as polyvinyl alcohol, starch, or styrene acrylic emulsions, among others. Surface sizing using an aqueous dispersion of a copolymer derived from at least one monomer of styrene or a styrene derivative, at least one monomer of an ester of ethylenically unsaturated carboxylic acids and alkanols such as C1-C4 alkyl acrylates and C1-C4 alkyl methacrylates, and at least one monomer of an ethylenically unsaturated carboxylic or sulfonic acid or salt thereof, is described by U.S. Pat. No. 6,087,457 as superior to surface sizing using polymer dispersions prepared from emulsion polymerization of styrenic monomers with alkyl (meth)acrylates. U.S. Pat. No. 5,139,614 discloses a polymer made from styrene and methacrylic acid for use in sizing paper to provide it with resistance to penetration of ink and aqueous liquids. U.S. Pat. No. 3,983,268 describes surface sizing paper by impregnating it with an aqueous solution containing a water soluble salt of a random copolymer based on styrene and a comonomer selected from acrylic acid, C1-4 alkyl esters of acrylic acid and methacrylic acid.
The above methods are concerned with rendering printing and writing paper sufficiently water-repellant so that there is minimal feathering of applied ink, thus allowing for more sharply printed or written lines. Generally, however, paper that is useful for machine printing, writing, or photocopying, and prepared by the above methods is not sufficiently water-repellant or waterproof so as to remain intact and legible when wet or provide a surface that, under wet conditions, can be written upon with pen or pencil.
Paper has been treated so as to not readily disintegrate when wet. As an example, a method for making waterproof photographic paper is known where a polyolefin composition is applied to both sides of the paper by extrusion coating. This method is described in U.S. Pat. No. 4,820,582 as one way to protect photographic paper from penetration by water and aqueous processing solutions. The aforementioned patent also recognizes the need to protect the edges of the paper from such penetration and suggests solving the problem by internally sizing the paper with a hydrophobizing agent comprising alkylketene dimers. However, photographic paper is generally not suitable as a substrate for writing with pen or pencil, for printing, or for photocopying.
Another example of paper that is highly resistant to disintegrating when wet is filter paper. U.S. Pat. No. 5,762,799 describes a method for making filter paper having water repellency, wherein the paper is impregnated with a copolymer based on n-butylacrylate-styrene. Again, however, such paper is not suitable as ordinary writing or printing paper, or as paper that can be used in photocopiers or laser printers.
Additionally, treatments have been described for imparting a degree of water-repellency to printable paper, sufficient to allow its use for security documents and currency. U.S. Pat. No. 5,660,919 describes making printable sheets for bank notes and other valuable securities by treating at least one side with a composition comprising at least one filler and at least one elastomeric binder that is an aqueous dispersion having polyurethane, acrylate copolymers and, optionally, carboxylated styrene-butadiene copolymers and polymers having acrylonitrile, isoprene, or neoprene monomers. The above methods, however, are not directed to paper that provides a surface for writing with pen or pencil in a wet environment, or that can be used in laser copiers or photocopiers and then used in wet environments.
A number of other methods are known for treating paper surfaces to provide water resistance thereto. U.S. Pat. No.4,537,595 describes spraying paper with an aqueous solution of sodium sulfide and an organopolysiloxane with Bunte salt groups, or with an emulsion of hydrogen siloxane followed by heating, to durably hydrophobize the paper. U.S. Pat. No. 6,054,020 describes applying amine-modified polysiloxanes to the outer surface of a tissue product to provide some degree of water or liquid repellency. One method for providing a moisture-proof coating on paper, while allowing the paper to be easily repulped and recycled is found in U.S. Pat. No. 5,695,608. The moisture-proof, film-forming synthetic resin used comprises plate crystalline phyllosilicate compound particles and a moisture-proofness enhancing agent such as organoalkoxysilane or polyamide polyurea compounds. The patent noted that it was well known that permeation of water in paper could be prevented by applying a coating layer of a hydrophobic film-forming resin such as polyethylene, polypropylene, and polyvinylidene chloride, but that such paper could not be repulped and recycled.
Methods that use wax to produce moisture-proof paper are also known. Japanese unexamined patent pub. No.50-36,711 describes such a method where paper is coated with an aqueous emulsion containing paraffin wax and then dried under heat. Japanese unexamined patent pub. No.56-148,997 discloses a composition used to moisture-proof paper sheets comprising a synthetic hydrocarbon resin and wax in water. U.S. Pat. No. 2,453,380 describes treating the inside of cardboard containers with latex, wax, and zinc stearate or oxide as a filler. Japanese unexamined Pat. Pub. 2000-80595, in describing a method for making water resistant printing paper by coating with a dispersion of a copolymer based on styrene and alkyl acrylate ester, mentions that mixtures of styrene acrylic resin and paraffin type wax have also been used.
So-called “barrier papers” that are resistant to water are the subject of U.S. Pat. No. 5,330,622. That patent, in recognizing that it was known to coat paper with latex-based compositions, cited French patent appl. A-2 365 002, which describes treating paper to make it water resistant by coating it with a composition having equal amounts of acrylostyrene latex and an aqueous dispersion of a metal salt such as calcium stearate. European patent appl. A-0 187 673 describes impregnating paper with a composition based on PVOH associated with an aldehyde such as glyoxal. U.S. Pat. No. 5,330,622 is directed to providing hydrophobic, oleophobic, and solvanophobic barrier properties to paper by impregnation with a composition comprising glyoxal, dimeric alkylketen, and a fluorinated salt. U.S. Pat. No. 4,110,155 also describes rendering paper waterproof by extrusion coating thereon or adhesively laminating thereto synthetic films such as polyolefins, polyesters, polyamides, and cellulose esters.
Various methods for using silicone-based materials to render paper more water resistant are known. U.S. Pat. No. 3,481,829 suggests using a hydrophobic organo-silicone such as a curable epoxy silicone resin prepared and applied as an aqueous emulsion. U.S. Pat. No. 3,085,902 discloses coating paper with colloidal silica and silicones to make it more water repellent. The colloidal silica is added to either the pulp or paper surface to increase silicon pickup. The paper is then coated with silicone as a water emulsion or solvent system. U.S. Pat. No. 2,507,200 claims to prepare highly water repellent paper by applying to its surface water soluble siliconates, such as alkali siliconates, then drying. The aforementioned patent characterizes this method as an improvement to older methods such as that described in U.S. Pat. No. 2,306,222 for waterproofing paper by contacting it with an organosilicon halide in vapor form, followed by contact with an alkaline reagent (i.e. ammonia) to neutralize the acid generated.
None of the above methods are directed to providing a paper for writing or printing that can, not only remain intact, but also maintain a surface that can be legibly written upon with pen or pencil under extreme wet conditions. Nor are any of the above methods directed to providing such paper than can also be used in photocopiers and laser copiers. Nor do any of the above methods provide a method for coating a wide variety of available, stock papers for writing or printing to impart thereto such characteristics.
A writing paper that can be written upon with an all-weather pen or pencil under wet conditions, and that can withstand prolonged exposure to such conditions without disintegrating has been provided in the past by the J. L. Darling Corporation of Tacoma, Wash. The weatherproof paper was prepared by coating stock writing paper with a proprietary solvent-based coating comprising a vinyl toluated alkyd resin.
There are a number of disadvantages, however, associated with using solvent-based materials for such applications. The cost of using solvent-based materials has greatly increased, largely owing to more strict environmental regulations, promulgated in recent years. Solvents require special handling and storage. Costly equipment is needed to recover or otherwise capture evaporated solvents, which are generally toxic. Longer, more expensive ovens may be needed for a more gradual drying process that avoids creating explosive mixtures. The cost of disposing of toxic and noxious solvent waste products can be considerable. Also, paper coated with solvent-based materials is generally not suitable for use in laser copiers or photocopy machines and, in many instances, is not repulpable or recyclable.
Accordingly, there remains a need in the art for paper that has been coated or otherwise treated with water-based materials to thereby render the paper usable under substantially wet conditions, such as might be encountered in areas exposed to inclement weather, where the paper can be repulped and recycled. More particularly, there remains a need in the art for such paper that maintains, when wet, a surface sufficiently intact and undisturbed so as to continue to legibly bear machine printed or written images. There also remains a need in the art for such paper having a surface that can be written upon with pen or pencil under wet or dry conditions. Further, there remains a need in the art for paper having such properties that is also usable as photocopy or laser printer paper. The paper should also be repulpable and recylable. Finally, there remains a need in the art for methods that can provide paper having the above-described properties by coating a number of commercially available paper stocks with water-based materials capable of imparting the requisite water resistance. The present invention fulfills these needs and provides further related advantages.